(1) Field of the Invention
This invention relates to photovoltaic elements useful for converting light and particularly for converting solar energy into electrical energy. The invention features the use of organic photoconductive materials.
(2) State of the Prior Art
Chief among materials used in the past for solar cells have been inorganic semiconductors, due to their fairly high conversion efficiencies which have been as high as 12 to 15%, for example, for silicon. However, such devices have proven to be very expensive to construct due to the melt and other processing techniques necessary to fabricate the semiconductor layer. As a result, such devices have had extensive practical utility only in the field of space exploration, and not in terrestrial applications.
In an effort to reduce the cost of solar cells, organic photoconductors and semiconductors have been considered, due to their inexpensive formation by solvent coating and similar techniques. However, prior-art organic materials have generally produced solar cells with conversion efficiencies only as high as about 0.05% at their highest, when exposed by incident sunlight at an intensity of 100 mw/cm.sup.2. An example of such a material is crystal violet, as described, for example, in U.S. Pat. No. 3,844,843. Still higher efficiencies are desirable if the cells are to have practical terrestrial use, notwithstanding their inexpensive cost of manufacture. An efficiency of 0.3% was reported as being achieved through the use of an undisclosed dopant, as noted in "Prospects for Direct Conversion of Solar Energy to Electricity," AWA Technical Review, Vol. 15, No. 4, 1974, reference 3, but none of the materials used has been disclosed.
Solar cells utilizing other organic photoconductive materials are disclosed in U.S. Pat. Nos. 3,009,006; 3,057,947; 3,507,706 and 3,530,007 and IBM Technical Disclosure Bulletin 18 (8), No. 2442 (January, 1976). However, there is no disclosure in any of these publications that the resultant solar cells exhibit a conversion efficiency high enough for extensive practical terrestrial use, i.e., greater than about 0.05%.
Photoconductive materials containing pyrylium-type dyes have been used in electrophotographic copiers. For example, U.S. Pat. Nos. 3,250,615 issued May 10, 1966, and 3,938,994 issued Feb. 17, 1976, describe pyrylium-type dyes useful in photoconductive compositions. Particularly useful pyrylium-type dye-containing photoconductive compositions are the so-called multiphase heterogeneous or "aggregate"-containing photoconductive compositions described in U.S. Pat. Nos. 3,615,414 and 3,615,415 both issued Oct. 26, 1971; 3,679,407 issued July 25, 1972; 3,706,554 issued Dec. 19, 1972; 3,732,180 issued May 8, 1973; and 3,873,312 issued Mar. 25, 1975. Certain photoconductors other than those multiphase heterogeneous compositions noted above have been successfully used in some instances in solar cells. However, U.S. Pat. No. 3,507,706 notes that it is well-known that, although certain photoconductors such as a charge-transfer photoconductor material are useful in electrophotography, this does not mean these same materials necessarily possess a utility in photovoltaic elements. This is demonstrated in part by the different physical and electrical properties of the photoconductive materials when used in the two different environments. Therefore, it cannot be ascertained with any degree of certainty whether a photoconductive composition useful in electrophotography will be useful in fabricating a solar cell and, if useful, whether any improvement in conversion efficiency can be obtained.
Aggregate photoconductors have been sandwiched between an electrode on one side and a charge-sensitive silver behenate layer on the other side to create an electrosensitive recording element as described, for example, in U.S. application Ser. No. 850,105 filed on Nov. 9, 1977, a continuation application of Ser. No. 731,861 filed on Oct. 12, 1976 by Light, entitled Electrophotothermographic Elements, now abandoned. However, the silver behenate layer is not an electrical conductor and the sandwich does not function as an effective solar cell.